Because most power sources have a limited range of output RPM, machines employing such power sources will also typically employ a multispeed transmission to provide a greater range of machine speed and acceleration. One type of multispeed transmission is the planetary transmission, but many other transmission types are known. While transmissions having a number of discrete gears are useful in allowing a wider range of operation, such systems generally entail certain disadvantages.
For example, because of the abrupt change of gear ratio at each shift point, any slack or stretch in the power train is allowed to relax or “unwind,” causing system sensors to provide readings that will result in a gear change back to the previous gear. This process may then repeat after a change to the previous gear, and so on, resulting in what is referred to as gear “hunting,” wherein the transmission executes rapid repeated shifts between adjacent gears. Gear hunting can cause a degraded operator experience, and also often results in unnecessary wear on the entire power train.
In order to prevent hunting and the consequent problems, a number of systems have been devised to preclude rapid shifts between adjacent gears. Of these, one solution has been to provide a shift timer. The shift timer is started at the time that a gear shift occurs, and a transmission controller precludes a subsequent shift until the timer runs out. However, such systems can cause engine lugging or over speeding in the event that a legitimate change of gear ratio is required within the timer duration due to a change in actual machine operating conditions.
To try to prevent lugging, certain solutions have been devised. For example, in U.S. Pat. No. 7,660,657, entitled “Anti-Hunt System and Method for Parallel Path Variable Transmission,” a system is disclosed for controlling a split torque transmission having a plurality of contiguous selectable operating ranges, with adjacent operating ranges being separated by synchronous points. After an upshift, the system sets a lock-out timer to prevent a subsequent shift of the transmission until the timer has expired, with the timer being overridden if the transmission output ratio displays certain trends.
While the '657 system effectively prevents high-frequency hunting based on trend tracking, the existence of drive train ringing or unwinding may in some cases hide a transmission output ratio trend. It may also be desirable to further limit the override of the lockout timer in some way to prevent lower frequency shift oscillations.
The present disclosure is directed to a system that may exhibit numerous distinctions over prior systems. However, it should be appreciated that any such distinction is not a limitation on the scope of the disclosed principles or of the attached claims except to the extent expressly noted. Additionally, the discussion of any problem or solution in this Background section is not an indication that the problem or solution represents known prior art except as otherwise noted by the inclusion of a patent number. In the case of the identified patent, the foregoing is intended as a brief summary, not as a construction of the patent in question or its claims, and the reader is referred to the patent itself in the event that a full and accurate understanding is desired.